The present invention relates to a method and a device for drying substrates.
European patent application 0 385 536 discloses a method and device for drying substrates after treatment in a liquid. According to the method known from this printed document, substrates are treated for a period of time in a bath that contains a liquid and are then removed so slowly that practically the entire liquid remains within the bath. In so doing, the substrates are brought directly from the liquid into contact with a vapor. The vapor does not condense on the substrate and mixes with the liquid whereby the mixture has a lower surface tension than the liquid. This method, however, is very complicated in practice because vapor is required and must be removed, and, in particular, supply lines and exhaust nozzles for the vapor must be made available.
From U.S. Pat. No. 4,722,752 a device and a method for cleaning and drying of disc-shaped substrates, for example, semiconductor wafers, is known in which the heat energy remaining in the substrates is used to generate a surface tension gradient. External heat is not supplied for improving or accelerating the drying process.
U.S. Pat. No. 4,920,350 shows and discloses a device and a method for cleaning and drying substrates whereby at the surface of the liquid energy in the form ultrasound is supplied to the substrate upon removal. The supplied energy however does not serve for drying but for cleaning.
The washing and drying method known from U.S. Pat. No. 5,368,649 for mechanical or electronic components as well as lenses employs for improvement of the drying method a liquid which is maintained at a pressure that is above atmospheric pressure. The rinsing liquid is heated above the boiling point defined at atmospheric pressure. For the drying process, the workpiece is introduced into a drying chamber in which a sudden decompression takes place so that the rinsing liquid on the workpiece evaporates quickly.
The invention has the object to provide a method in which the substrates during removal from the fluid bath can be dried quickly, at minimal expenditure, without impacting the environment, and without residues forming on the substrates.
The inventive objective is realized with a method of the aforementioned kind in that during withdrawal of the substrate from the liquid, a liquid meniscus that is formed at the transition between the substrate surface and the liquid surface is heated.
One advantage of the present invention is that with the inventive process the substrate is very rapidly dried, without great technical expenditure, such drying being very thorough and without mechanical stressing during the withdrawal of the substrate from the liquid bath.
Pursuant to one specific embodiment of the invention, electromagnetic radiation is used for heating the meniscus of the liquid. Such radiation has the advantage that it can be generated very easily, passes through certain materials, such as a hood over the liquid bath, and can be very easily focused upon the meniscus of the liquid.
The electromagnetic radiation preferably comprises microwaves, infrared radiation, and/or visible radiation.
According to a very advantageous embodiment the meniscus is heated by a gas whereby a heat exchange from the gas onto the meniscus takes place. This provides for a simple, efficient heating of the meniscus whereby, preferably, at least one hot gas stream is generated and directed onto the meniscus and/or the meniscus is surrounded by a hot gas that heats the meniscus. In the latter case, convection of the gas is advantageous. The local heating of the meniscus effects a reduction of the surface tension within the curved meniscus area that forms upon removal of the substrate between the substrate surface and the surface of the liquid. In this manner, the liquid present within the meniscus, respectively, adhering to the substrate surface, is moved away from the meniscus, respectively, away from the substrate surface so that the substrate can be quickly and reliably freed of the liquid and dried substantially without additional measures during lifting from the liquid. The use of hot gas as an energy transmitting medium for heating the meniscus provides an especially simple and advantageous possibility for heating the meniscus.
For heating the meniscus of the liquid, hot nitrogen is advantageously used that is guided to the liquid surface such that the liquid in the area of the meniscus experiences a greater temperature increase then the liquid area outside of the meniscus. In doing so, the gas may be stationary as well as in the form of a gas flow. The gas is preferably introduced via a hood that can be placed above the liquid.
It should be noted that heating, in general, refers to a heating of the meniscus relative to the other areas of the liquid whereby these temperature differences can be optionally small and even within the magnitude of less than 1xc2x0 C.
Preferably, the heating gas is an inert gas in order to avoid reactions of or chemical effects on the substrates caused by the gas. As a heating gas nitrogen is advantageous, but noble gases are also well suited. The gases are preferably not miscible with the liquid.
According to one very advantageous embodiment of the invention, a gas is employed that reduces the surface tension of the liquid from which the substrates are removed. This effect, known as the Marangoni effect, contributes additionally to an excellent and fast drying of the substrates. Preferably, the gas for reducing the surface tension of the liquid is nitrogen, isopropyl alcohol, or a gas mixture containing at least one of theses gases. The gas for reducing the surface tension of the liquid is preferably also used as the heating gas for heating the meniscus. The heating gas is directed onto the meniscus advantageously via a hood which can be placed above the liquid container from which the substrates are to be removed. Examples for introducing the gas via a hood are disclosed in conjunction with introduction of gases for reducing the surface tension of the liquid in German patent application 44 15 077, assigned to the assignee of the instant application and incorporated by reference into the present application.
According to an alternative or additional method variation, the object of the invention is also solved in that the meniscus is heated by the substrate that contacts the meniscus, whereby the substrate or substrates before and/or during removal from the liquid are heated. The heating of the meniscus by the substrate, which is brought to a temperature greater than the temperature of the liquid, provides a further very advantageous solution to the inventive object.
In connection with the heating of the meniscus by the heated substrate, it is especially advantageous when before and/or simultaneous to the heating of the meniscus a vapor or gas, that mixes with the liquid of the meniscus, is brought into contact with the meniscus. In this manner, it is possible to additionally decrease the reduction of surface tension, resulting from heating of the meniscus, by the Marangoni effect and to thus improve and accelerate drying.
The inventive object is also solved in regard to a device for drying of substrates with an energy source for heating a meniscus, which forms upon removal of the substrates from the liquid at the transition between the substrate surface and the liquid surface, in that the energy source is a hot gas that surrounds the meniscus or flows onto the meniscus. In the first case, the device has a heating device that heats the gas surrounding the meniscus. As an example of such a heating device, infrared or ultrasonic energy sources are advantageous, but also eddy current heaters.
A further solution of the inventive object, as an alternative or in addition to the aforementioned devices, is that the energy source for heating the meniscus is the substrate itself which during removal of the substrates is warmer than the meniscus. The substrate is preferably heated before and/or during removal from the liquid.
Preferably, the inventive methods and devices are employed in connection with drying of semiconductor wafers, whereby the efficiency of the drying process and its reliability are improved and the drying period is decreased so that the productivity of the inventive method and of the inventive device is increased.
Pursuant to a further preferred specific embodiment of the invention, when the substrates are withdrawn from the liquid bath they are brought into direct contact with a vapor that does not condense upon the substrate and contains a substance that is miscible with the liquid and, when it is mixed with the liquid, results in a mixture that has a lower surface tension than does the liquid. As a consequence of the combination of the inventive method with the method known from the aforementioned European patent application 0 385 536, there results for particular applications an additional improvement and acceleration of the drying of the substrates.
The substrates are advantageously lifted and lowered by a blade-like lifting mechanism. The blade-like lifting mechanism carries the substrates, when they are raised out of the liquid, and in particular at those locations of the substrate that are the last to leave the liquid. Drops of the liquid then flow off the substrate over the blade-like edge of the blade-like lifting mechanism, so that when the substrate leaves the liquid bath, at which point a meniscus is no longer defined, the drying process can continue at the same efficiency.